1. Field of the Invention
The present invention pertains to radiation backscatter measurement of particulate material and, more particularly, to method and apparatus using a gamma-ray backscatter technique for determining incombustible content of coal dust and rock dust mixtures in coal mines.
2. Discussion of the Prior Art
The major source of energy in disastrous coal mine explosions is the explosive combustion of coal dust. Such explosions usually are initiated by the explosion of a relatively small amount of methane which blows coal dust lying on the floor or walls of the mine into the air and simultaneously ignites it. The resulting secondary explosion blows still more coal dust into the air and ignites it, and the explosion is thus propagated throughout the mine or until an area is reached where insufficient fuel is available to maintain combustion.
One way that such explosion propagation can be prevented is by mixing rock dust, such as limestone or dolomite, with the coal dust in sufficient quantity to produce a mixture containing 65% or more of inert, incombustible material. The Coal Mine Safety Act of 1969 requires all mine operators to maintain such a minimum incombustible material concentration in all dust found underground undergound coal mines at distances greater than 40 feet from a working face.
It becomes necessary, therefore, to provide a method for determining the amount of incombustible content in coal mine dust, both from the operator's standpoint of knowing when sufficient rock dust has been applied and from the mine inspector's viewpoint during a mine inspection. At present, two methods, volumetry and, much less commonly, a chemical or ashing method, are used to measure the amount of incombustibles in coal mine dust. The more commonly used method, volumetry, is a very simple liquid-displacement technique for determining the specific gravity of a 20-gram sample of the mixture of coal and rock dust. Although reasonably accurate, this method has the disadvantages of being time consuming and requiring a considerable amount of equipment and reagent; and, furthermore, this method has the greater disadvantage of being essentially a laboratory technique thereby requiring that samples be collected underground, packaged and transported to the mine entrance, often several miles away, for shipment to a laboratory. Obviously, a rapid, accurate, instrumental method capable of use in situ would be most desirable for determining the percentage of incombustible material in coal mine dust, not only to decrease the time required for a sample to be analyzed by an inspector, but also to permit hazardous conditions to be detected and corrected in as short a time as possible.
Experimental work with the use of gamma-ray backscatter and transmission absorption techniques for determining coal ash content has been reported by several investigators in the past. A preliminary investigation by J. W. Martin and R. F. Stewart, "Determination of Incombustible Content of Mine Dust By Nuclear Method," BuMines RI 7193, 1968, showed that, using an americium-241 source, roughly a 30% change in counting rate resulted from a 35% change in incombustible content in the range of interest, 50 to 80% total incombustible content. This measurement was performed using a flat sample surface of "infinite" thickness, about 4 inches for americium-241; however, because a sufficient sample is seldom available to provide an infinite thickness in practice, a different approach was required.
The use of scattered gamma-rays to determine shale and ash in coal has been studied in the past with the result that it was determined that the use of low energy (below 100 kev) gamma-rays was necessary to obtain sufficient difference in scattered characteristics of carbon and those of heavier elements to accomplish the desired purpose. Thulium 170 with an 85 kev gamma-ray was used for an irradiation source in these studies; and, as reported by L. Hardt, "A Rapid Method For Determining The Ash Content of Coal By Means of Low Energy Radiation," Paper B3, 4th International Coal Preparation Congress, England, published by National Coal Board, pp. 101-108, 1962, and Martin and Stewart, mentioned above, as well as others, the sample thickness was required to exceed three inches. Hardt suggested six inches and Martin and Stewart suggested four inches dependent primarily on the isotope (and consequent gamma-ray energy) employed. Accordingly, the use of a flat scattering surface imposes a minimum sample size or, in the case of in situ measurements, a minimum sample thickness and, therefore, has the disadvantage of requiring large samples or thick beds of dust.
An additional problem involved in the measurement of incombustibles content of coal mine dust is that of variation in bulk density of the sample. It has been discovered that at selected spacings of the radiation source sample and detector, a combination could be achieved at which the backscatter measurement was uneffected by changes in bulk density, as exemplified by U.S. Pat. No. 3,505,520 to Stewart et al, but the use of such a combination has the disadvantage that it is effective for only a single particular incombustible composition and introduces error for other compositions.
While the above described prior art systems have been found satisfactory for laboratory studies, considerable modifications thereof would be required to produce a satisfactory field instrument for in situ use. The basic problem to be overcome stems from the fact that although the source-detector separation technique for bulk density compensation is operable for a given sample composition, it actually exaggerates the problem with different compositions.